The invention relates to a photometric circuit for a camera, and more particularly, to a photometric circuit of a so-called photocurrent direct integrating type in which a photocurrent produced by a photoelectric transducer element, which is used for purposes of photometry, is directly integrated by an integrating capacitor.
As is well recognized, a photometric circuit which utilizes an integrating capacitor for directly integrating a photocurrent produced by a photoelectric transducer element and which utilizes the integral for purpose of automatic exposure control or the like is extensively used in cameras of film surface reflection photometry type. As compared with the so-called logarithmic compression photometric circuit in which a photocurrent is initially converted into a logarithmically compressed voltage, the photometric circuit of direct integrating type has the following features:
1 The circuit arrangement is simplified, and avoids the need for any temperature compensation.
2 A good linearity in the integrating response permits a real time tracking of a varying photocurrent over an extensive range, by properly constructing an integrating amplifier. This assures a high accuracy in integrating a high brightness input such as may be produced by flashlight from an electronic flash or in integrating a low brightness input which involves an exposure over a prolonged period of time.
3 A response lag upon power on is negligibly small as compared with the circuit of logarithmic compression type, and this eliminates the need for a special compensation circuit.
On the other hand, the photometric circuit of direct integrating type has the following drawbacks as compared with the circuit of logarithmic compression type:
1 A dynamic range of photometry is reduced.
2 Since the photometric output can only be derived in the form of an integral output, information which may be used for other purposes such as display, for example, the magnitude of photocurrent, cannot be directly obtained.
On the other hand, in the conventional photometric device utilizing a reflection photometry technique, the reflectivity of the film surface is presumed to be constant irrespective of the variety of films, and the surface of a first shutter blind is worked to provide a reflectivity which is substantially equal to the reflectivity of an average film surface so that the photometry of reflected light from both the first blind surface and the film surface enables an exposure control signal to be derived.
However, in practice, the reflectivity of the film surface varies as the variety of the film is changed, and where a picture is taken using a film having a reflectivity which differs from the reflectivity of the average film, a proper exposure control cannot be achieved. In addition, a proper exposure control is also precluded where the reflectivity of the first blind surface or the film surface varies or fluctuates.
To overcome these difficulties, there has been proposed several reflection photometry devices (see Japanese Laid-Open Patent Applications No. 46,725/1978 and No. 151,029/1979), which comprise a first photometric circuit used for exposure control and a second photometric circuit used for correction purposes. During running of the first blind or in the course of the exposure, the second photometric circuit determines reflected light from the first blind which moves across the image field as well as reflected light from the film separately, and forms a difference between the photometric signals in order to provide a correction of an output from the first photometric circuit.
However, these reflection photometry devices require (i) a photometric circuit which is used to provide a correction to thereby result in a complex circuit arrangement, and (ii) involve an error between the two photometric systems, which prevents a correction of a high accuracy from being achieved. The error is caused by differential locations where the photoelectric transducer elements of both photometric circuits are disposed to thereby change the photometric distribution of both elements with respect to the image field. Such error cannot be completely eliminated as long as the two photometric systems are employed.